/Engine/Source/Runtime/Engine/Classes/Materials/MaterialExpressionCustom.h
virtual bool GenerateHLSLExpression ( [FMaterialHLSLGenerator](API\Runtime\Engine\FMaterialHLSLGenerator) & Generator, [UE::HLSLTree::FScope](API\Runtime\Engine\HLSLTree\FScope) & Scope, int32 OutputIndex, [UE::HLSLTree::FExpression](API\Runtime\Engine\HLSLTree\FExpression) const *& OutExpression ) const